1. Technical Field of the Invention
The present invention relates to a method and apparatus for reforming a hydrocarbon-based fuel, alcohol, etc. into a fuel gas containing hydrogen, for industries which use high-purity hydrogen as a fuel, such as for fuel cells.
2. Prior Art
When electric power is generated using fuel cells, hydrogen is supplied to the fuel cells; a fuel gas containing hydrogen is produced from a raw material consisting of hydrocarbon based fuels, e.g. butane or propane, or alcohol based fuel such as methanol; the raw material is reformed in a reforming vessel containing a catalyst, in which a mixture of the fuel gas, steam and air is reformed.
The reforming reaction proceeds at a rather high-temperature and heat is absorbed during the reaction, so when a conventional reforming device is used, the mixed gas is heated sufficiently in a preheater, and using the heat retained in the gas, the temperature of the catalyst is increased, or is otherwise heated by an external means, so as to expedite the reforming reaction.
Recently, a self-heating system is currently used for a reforming device. In the self-heating system, a mixed gas and reforming catalyst are heated by the oxidation of a part of the mixed gas and the gas is reformed by the heat.
If a gas mixture is supplied to one end of a reforming device filled with a partial oxidation catalyst and a reforming catalyst, and if the reformed gas is discharged from the other end after the gas mixture has made contact with the partial oxidation catalyst and the reforming catalyst, then only the upstream portion of the reforming catalyst near the partial oxidation catalyst is over-heated, and the temperature of the downstream portion of the reforming catalyst, located further away from the partial oxidation catalyst increases after a time delay. As a result, the temperature distribution of the reforming catalyst is uneven, therefore, a fairly long time is required before the temperature of the entire reforming catalyst has been increased, so the reforming device cannot be started up quickly.
In addition, because part of the reforming catalyst is over-heated due to the uneven temperatures distribution, deterioration of the catalyst, such as sintering occurs.
Recently, a new reforming device has been developed and is in practical use; the partial oxidation catalyst and the reforming catalyst are installed in multiple layers, so as to distribute the temperature increase of the reforming catalyst more evenly. This type of reforming device is typically classified into the series type shown in FIG. 1, and the parallel type in FIG. 2.
In the series-type reforming device, the reforming room is arranged in multiple stages (3 stages in FIG. 1) and each stage has a partial oxidation catalyst on the upstream side and a reforming catalyst downstream, and a gas mixture containing a fuel vapor such as methanol, steam and a small amount of air is introduced at one end of the device, and the reformed gas is discharged from the other end. To expedite the partial oxidation reaction of the gas mixture, additional air is fed into the second and third reforming rooms. In the series-type reforming device, the temperatures of the reforming catalysts in each stage are increased automatically by the heat of the partial oxidation reaction, and the length of the passage in which the gas mixture contacts the reforming catalyst can be made long, so the advantage of a high reforming rate can be expected.
Conversely in the parallel-type reforming device, partial oxidation catalysts and reforming catalysts are arranged in a number of stages (3 stages in FIG. 2), in the same way as with the series-type device, and each stage is separated from the others, and a gas mixture containing a fuel vapor such as methanol, steam and a small amount of air is supplied to each stage, and a reformed gas is discharged from each stage. Also with this parallel-type device, the temperatures of the reforming catalysts in each stage can be increased evenly using internally generated heat, and because only the gas mixture is distributed to each stage of the reforming device, the construction can be simplified which is an advantage. If part of the reforming catalyst etc. deteriorates accidentally, each stage can be quite easily replaced individually, which is also an advantage.
However, the aforementioned series- and parallel-type reforming devices are accompanied with the following problems.    (1) With the series-type reforming device, air must be supplied to the reforming rooms at the second and subsequent stages from an external source, so the air piping is complicated and requires a dedicated space. The air supplied from outside must be mixed completely with the gas mixture in the small space between adjacent reforming rooms and then fed to the reforming rooms, but this space is normally small, so the mixing often becomes incomplete. As a consequence, inappropriate reactions may sometimes take place, for example, irregularities may occur in the partial oxidation or reforming reactions.    (2) With the parallel-type reforming device, on the contrary, since the fuel mixture such as methanol, steam and air is mixed completely beforehand and then fed to each reforming room, the problems mentioned above for the series-type reforming device do not occur. However, as the length of the passage in which the gas mixture contacts the reforming catalyst is short, the necessary reforming rate may not be obtained when the distribution of reforming catalysts or the distribution of carrier materials are not maintained evenly.
When a reforming device is used for fuel cells in an electric automobile etc., the motor must be started quickly by generating electric power by supplying high-purity hydrogen into the stack of fuel cells as quickly as possible. The device must also be as compact as possible.
However, with a conventional self-heating system of series- or parallel-type reforming devices, compactness of the device is inconsistent with a high reforming rate as described above.
The hydrogen, required to generate electric power in a fuel cell, is produced by a reforming reaction using a raw material consisting of either a hydrocarbon based fuel, such as butane and propane, or an alcohol based fuel, such as methanol. However, because the hydrogen-rich reformed gas produced by the reforming reaction contains a large amount of carbon monoxide (CO) as an impurity, it should be removed before supplying it to a fuel cell that requires high-purity hydrogen. This is because if CO is fed into the fuel electrode of the fuel cell, it is adsorbed by the catalyst in the electrode, poisons the catalyst, decrease the reaction at the electrode, and lowers the electricity-generating performance.
Under these circumstances, the reforming device is normally provided with a CO removal unit filled with a CO removing catalyst, where a selective CO oxidation reaction (CO+1/2O2→CO2) or, if required, a CO shifting reaction (CO+H2O→CO2+H2) occurs, thus the concentration of carbon monoxide is reduced, in this additional mechanism.
With a reforming device that produces hydrogen-rich reformed gas from a hydrocarbon-based fuel or an alcohol fuel, the reforming reaction proceeds endothermically, so heat must be supplied to the reforming unit. In addition, it is also important to supply heat to increase the rate of the reforming reaction. Therefore, in many cases, fuel gas, water and air are heated by an external heat source to a temperature appropriate for the reforming reaction, to produce a high-temperature vapor which is then fed to the reforming unit, or the gas mixture is heated up to such a temperature in the reforming unit where the reforming reaction takes place.
On the other hand, a CO removal unit containing a catalyst mainly intended to decrease the concentration of CO contained in the reformed gas produced in the reforming unit. the selective CO oxidation reaction begins at about 100 to 200° C. and a CO shift reaction occurs at about 200 to 300° C. In addition these reactions are exothermic, the temperature of the CO removal catalyst should be prevented from increasing in order to obtain a high CO removal rate. For this reason a conventional reforming device of the reforming unit must be designed to be seperate from the CO removal unit, or if an integrated design is used, a thermal insulation material is required to prevent the heat transfer from the reforming unit to the CO removing unit, and a method of cooling the CO removal unit should be used.
Furthermore, carbon monoxide created in the reforming reaction poisons the electrode catalyst in the fuel cell as described above, and interferes with the reaction of the electrode, so it should be removed from the reformed gas by a CO removal reaction. However, since the CO removal reaction is exothermic, if heat is transmitted from the reforming unit to the carbon monoxide removal portion (CO removal unit), the CO removal reaction does not proceed.
Consequently, in an integrated reforming device composed of a reforming unit and a CO removal unit, the heat transfer from the reforming unit to the CO removal unit must be decreased and the loss of heat from the reforming unit at high operating temperatures must be prevented.
Conventionally, the reforming catalyst is installed in a single cylindrical or square vessel, therefore when the device generates a large output, the sectional area of the passages in the catalyst vessel is also large, often resulting in an irregular distribution of fuel gas flow in the catalyst vessel, and a satisfactory reforming reaction is often not achieved.
When the reforming unit is constructed with the reforming catalyst installed in a single catalyst vessel, if even part of the catalyst deteriorates as a result of operating with an unbalanced flow of the gas mixture, the whole reforming unit must be replaced.